My parents would hord, yard sale things, their parents stuff, other stuff. When I was younger I had lots of baseball cards, games, whatever.
I think being forced into tough situations had me reevaluate what was necessary. So being forced to live on 3 days of clothes for 5 years helped. Now I delete photos regularly, Google accounts, text conversations and contacts - if they truly are important, you'll remember them.
As for how I determine what I don't need, having someone or an organization to give them to helps the mind a lot. Knowing that possibly someone else could actually like them. So, donation places for clothes, eBay for collectables, trash for junk honestly.
These days I simply don't buy things. For every thing you move on from and do not replace, that's more space where you live makes the room feel better.
So yeah a combination of feelings good about giving them to other people, and selling things so that I can buy more relevant and efficient things, and trashing things that serve no purpose and are not sentimental, and then for sentimental useless things just put in a box.
I find categorizing them first like that helps. Don't have to do it all in one day. Maybe take the month to really analyze what category they belong in
You'll feel better about the decision you make if you know you put a lot of thought into it.